1. Field of the Invention
The present invention relates generally to a method and apparatus for determining lane count error, if any, in a navigational system such as a range-range or hyperbolic system. Through the described method and apparatus of the present invention, an operator can economically determine whether a radio navigational system is giving erroneous positional information because it has lost or gained an integer lane count or counts and can determine the correct lane count, if there is an error.
2. Description of the Prior Art
Many ships and vessels utilize a radio or acoustic navigational system to determine accurately their position. Two major types of navigational systems are now in use: range-range systems and hyperbolic systems. Both of these systems are well-known in the art, and each includes a plurality of shore-based stations placed at precisely known locations and a navigational instrument (often a receiver and calculator) on board the vessel. The shore-based stations and the vessel-based instrument transmit and/or receive signals. In some systems the shore-based stations and the vessel-based instrument both transmit and receive signals from each other. In other systems, only the shore-based stations transmit signals; the vessel-based instrument only receives the signals and performs calculations based upon the reception.
All of these systems ultimately give the position of the vessel in an integer and fractional lane count format. For example, the system might give a reading of 140.13 from station no. 1 and 167.25 from station no. 2. Each lane count represents a known distance defined by the repetition rate of the signal and the speed of the signals. Only the fractional lane count can be directly determined by the shipboard receiver. As will be described more fully below, the integer lane count identifies the particular lane the vessel is in, and the fractional lane count describes exactly where within a given lane the vessel is located.
In both the range-range and hyperbolic navigational systems, the shipboard receiver constantly interprets the signals. As the vessel proceeds from point to point, the receiver counts the lanes and keeps track of the integer lane count. Therefore, as long as there is no interruption in the signals, the radio navigational system provides extremely accurate position information for the vessel. The integer lane count information tells the operator the lane box in which the vessel is located, and the fractional lane count information precisely defines the position of the vessel within that box.
By their nature, radio and acoustic ranging systems frequently include an ambiguity in the integer lane count. The signals of these systems repeat at short intervals of a few seconds to a few microseconds. During offshore navigation, it is not unusual for radio reception to be lost for a short period or for the signal to be shifted out of phase. For example, the reception or phase angle might be lost or shifted because of weather conditions or interference with large metal masses or skywave propagation with the radio signals. Once reception is lost, the shipboard receiver is no longer certain of the integer lane count. For example, if the signals repeat every ten microseconds, then the receiver on the vessel can determine that the lane count might be 120.37 microseconds, or 130.37 microseconds, or 140.37 microseconds, and so forth. It cannot, however, distinguish among these possibilities. This uncertainty is commonly known as "lane ambiguity." Thus, the navigational system can always determine precisely where the vessel is in a "lane," but it cannot accurately determine in which of the many possible adjacent lanes the vessel is located, if the radio or acoustic signal is ever lost.
Once "lane ambiguity" occurs through the loss of lane count, accurate navigation cannot be continued with range-range and hyperbolic navigational systems until the vessel can determine its position by some other means. Frequently the actual position of the vessel is determined by returning the vessel to shore or to a marker buoy which has been placed at a known location in the ocean. A return trip to shore requires considerable and expensive time and travel, and marker buoys are difficult and expensive to deploy, locate and retrieve. Other naviqation techniques, such as satellite and sextant naviqation systems, are qenerally not sufficiently precise to determine the specific lane in which a vessel is located.
When a vessel is being used for geophysical surveys, rig positioning, hazard surveys, hydrographic surveys, and similar applications, it is essential that the precise position of the vessel be known throughout the operations. It is further important that time and money not be lost because of a need to correct lane count error by conventional methods.